The present invention relates to Functional Electro Stimulation FES) systems and more particularly to an improvement to known methods for providing control signals to an FES system. Such control signals are required at a high update rate to provide effective stable control of a patient""s movement. The improvement provided by the present invention allows for a high effective update rate while reducing the overall data transmission rate.
Paraplegics and quadriplegics often have muscles capable of functioning, but are paralyzed due to damage to nerves that carry impulses to the muscles. Functional Electro Stimulation provides paraplegics and quadriplegics with use of their muscles by providing artificial stimulation pulses to the patient""s muscles, which stimulation results in a desired movement.
Prosthetic devices have been used for some time to provide electrical stimulation to excite muscle, nerve or other cells. Some of these devices have been large bulky systems providing electrical pulses through conductors extending through the skin. Disadvantageously, complications, including the possibility of infection, arise in the use of stimulators which have conductors extending through the skin.
Other smaller stimulators are implanted which are controlled through high-frequency, modulated RF, telemetry signals. An FES system using telemetry signals is set forth in U.S. Pat. No. 4,524,774, issued Jun. 25, 1985 for xe2x80x9cApparatus and Method for the Stimulation of a Human Muscle.xe2x80x9d The ""774 patent teaches a source of electrical energy, modulated by desired control information, to selectively control and drive numerous, small stimulators disposed at various locations within the body. Thus, for example, a desired, progressive muscular stimulation may be achieved through the successive or simultaneous stimulation of numerous stimulators, directed by a single source of information and energy outside the body.
Many difficulties arise in designing RF powered implanted stimulators which are small in size, and in passing sufficient energy and control information to the stimulators to satisfactorily operate them without direct connection. A design of a suitable, small stimulator, a microstimulator, which can be easily implanted, such as by expulsion through a hypodermic needle, is taught is U.S. Pat. No. 5,324,316 issued Jun. 28, 1994 for xe2x80x9cImplantable Microstimulator.xe2x80x9d The ""316 patent teaches all the elements required for successful construction and operation of a microstimulator. The microstimulator is capable of receiving and storing sufficient energy to provide the desired stimulating pulses, and also, is able to respond to received control information as to pulse duration, current amplitude and shape. Further, the stimulator of the ""316 patent achieves a xe2x80x9ccharge balancingxe2x80x9d, that is, a balancing of current flow through the body tissue in both directions to prevent damage to the tissue which results from continued, preponderance of current flow in one direction. The ""316 patent in incorporated herein by reference.
The microstimulator of the ""316 patent requires a control signal comprising the amplitude, width, and frequency of the desired stimulation pulse. In order to achieve effective control of muscles, to move the patient""s body in a desired manner, a pulse parameters update rate of approximately 100 Hz is required. Additionally, a plurality of microstimulators may be required to stimulate difference nerves or muscles in a coordinated manner to achieve the desired motion. The requirements for both transmission of the signals by a master control, and receipt of the signals by a multiplicity of microstimulators results in increased power consumption and potentially high error rates. What is therefore needed is a method for reducing the data rate of data that must be transmitted to achieve effective functional control of the patient""s muscles.
The present invention addresses the above and other needs by providing a Functional Electro Stimulation (FES) system utilizing an initial control message, which initial control message provides a complete set of stimulation parameters for a microstimulator, followed by a sequence of high data rate control messages, which high update rate control messages provide a single stimulation parameter for the microstimulator. As a result, effective stimulation is achieved at a much lower data transmission rate than when a complete set of stimulation parameters is transmitted at the high update rate.
In the natural use of human muscles, physiological stimulation is achieved by a change of frequency of the pulse sent to a muscle. In FES systems, stimulation levels may be controlled by changing pulse magnitude, pulse width, or pulse frequency. In a preferred embodiment, the pulse magnitude, pulse width, and pulse frequency are initially set to neutral values, which neutral values may subsequently be adjusted when needed, but changes to the level of stimulation to obtain muscle contraction is achieved by controlling pulse frequency only.
In accordance with one aspect of the invention, the control of pulse frequency is achieved by sending frequency change commands at a 100 Hz transmission rate. A typical range of pulse frequencies is from 10 to 200 Hz. The preferred step size of frequency changes is xc2x18 Hz or xc2x116 Hz. If a frequency change larger than the step size is desired, the change may be achieved over several commands. Advantageously, limiting the size of the command to xc2x116 results in a short message length and thus facilitates high command transmission rates.
It is a further aspect of the invention to provide feedback of stimulation parameters from the microstimulators. The feedback is used to verify that the previous control messages have been correctly received. If an error is identified, the stimulation parameters may be re-initialized, or alternatively, the high data rate control message may be used to correct the error.
It is a feature of the invention to provide a Mode in the control message, which Mode indicates whether the message is the initial control message or the high data rate control message. Based on the value of Mode, the microstimulator either processes the message as the initial control message or as the high data rate control message.
It is an additional feature of the invention to provide an ID in the control message, which ID indicates which of a multiplicity of microstimulators the control message is directed to. Based on the ID contained in the message, each microstimulator either processes the message or ignores the message.